In continuous casting, reaching a high casting speed and therefore attaining an always higher productivity, while still maintaining both the surface and internal quality of the cast product high, is connected to the optimization of a plurality of technological parameters relating both to the characteristics of the crystallizer and to the equipment connected to it, and also to the casting method.
Said parameters principally concern the geometric and dimensional characteristics of the crystallizer, the primary cooling system, the lubrication system of the internal walls and the material the crystallizer is made of.
Such parameters affect the capacity of the crystallizer to support the high thermal and mechanical stresses and the wear to which it is subjected, thus determining its operating life in conditions of great efficiency.
As far as the primary cooling system is concerned, in the known type of crystallizers, the high temperatures reached by the internal walls, in particular in the zone around the meniscus, significantly condition the tensional and deformational state of the crystallizer, considerably limiting the casting speeds that can be obtained because of the plastic deformation of the crystallizer and of the consequent drastic reduction in its working life.
Moreover, the variation in the thermal flow in the casting direction, which has a peak in correspondence to the zone of the meniscus, makes the temperature not uniform along the crystallizer, thus causing a non-homogenous deformation state, with subsequent problems connected to the defects in shape which this deformation causes on the cast product and to the premature wear of the crystallizer, which reduces its useful life.
A further problem is connected to maintaining the crystallizer in conditions of efficiency for long periods before having to resort to maintenance and/or replacement, deriving in particular from localized cracks in the zone of the meniscus caused by tensions and plastic deformation accumulated during the heating cycles.
In the crystallizers currently used it has been impossible to find a satisfying solution to all these problems, and indeed the attempt to solve them has instead led to accentuate others.
Thus, for example, in the attempt to increase the casting speed an unsatisfactory cooling of the product being made was obtained, and therefore the solidification of an insufficient thickness of the skin, with subsequent problems of breakage of the skin outside the crystallizer.
On the other hand, when it was tried to obtain an optimal cooling of the product, this entailed a reduction in the casting speed and therefore a reduction in productivity.
The document DE 4127333 describes a tubular crystallizer in which some channels, made in the walls and in which the cooling fluid circulates, are divided into parts in the zone astride the meniscus, by inserting little tubes of various sizes which divide the passage section.
The document US 2004256080 describes channels for the cooling liquid which have a smaller cross section in the upper zone and larger in the lower zone.
However, these documents do not describe any quantitative or qualitative criterion to identify the proportion between the channels with a larger section and channels with a smaller section, and/or their disposition, in the zone astride the meniscus.
The present invention thus proposes to supply an answer to these problems, looking for a solution which allows, in the first place, to increase the working life of the crystallizer in conditions of high casting efficiency, also bearing in mind the need to maintain as unchanged as possible the internal shape, with its substantially conical profile.
One purpose of the present invention is therefore to give the crystallizer a primary cooling system which allows to reach high casting speeds and at the same time allows to obtain a high number of casting cycles, so as to increase the working life of the crystallizer in conditions of great efficiency.
A further purpose of the invention is to reduce the peak value of the heat flow in correspondence to the zone of the meniscus so as to render as uniform as possible the development of the temperature along the crystallizer, allowing to maintain its shape unaltered, thus giving benefits in the quality of the final product and its casting ability, and to reduce the tensional and deformation condition with the advantage of a longer working life of the component.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other advantages, in particular a considerable increase in the working life of the crystallizer.